This invention aims at minimizing the electronic circuitry required for measuring a given set of temperatures via RTDs, thus lowering the cost while increasing the reliability of the measurement system.
The most common RTD for high accuracy applications is presently the platinum resistance element with three connecting wires (FIG. 1). The three wires are used in order to make the output V.sub.o of the basic bridge circuit (FIG. 1) in which this element is commonly utilized less sensitive to the length of said connecting wires. By adding more resistors to the basic bridge circuit it is possible to approximate a linear relation between V.sub.o and RTD temperature. Several of these bridge circuits may then be scanned; however a differential type of analog multiplexer is required. Due to the continuous flow of current through the RTD it is also necessary to keep the current value small in order to minimize self-heating. The result of said prior art concept is a low bridge signal output which places further requirements on the stability of the scanning and amplifying circuitry in order to provide any meaningful results.
In overcoming the deficiencies of the presently known measuring concepts, a number of desirable objectives are immediately apparent and obtainable in the concepts of the present invention.
The inventive concept of the present invention does not impose special requirements on the RTD construction but uses the existing widely available three wire configuration.
Elimination of the need of highly stable resistors heretofore required in the basic bridge circuit.
The elimination of any circuit non-linearity by use of constant current excitation so that the only non-linearity present in the voltage developed across the platinum resistance element, versus temperature, is due to the physical properties of the platinum itself.
By the use of a constant current in the form of large amplitude short duty cycle pulses, minimizing of self-heating is realized while relaxing the stability requirements of the signal processing circuitry.
And, a direct relationship of resistance versus temperature is obtainable by the use of a single ended analog multiplexer instead of a differential type.